One of the problems encountered by lens designers is that of astigmatic error. This refers to astigmatic errors caused by ophthalmic lenses themselves, as opposed to an astigmatism that exists as part of a patient's uncorrected vision.
Astigmatic error can be found in almost any ophthalmic lens, including single-vision lenses used to correct nearsightedness. It is a particularly significant problem in multifocal lenses, including progressive addition lenses (PAL's), which are a special case of such lenses.
Progressive addition lenses for correction of presbyopia have been the subject of extensive research and development over the past five decades, as reported in the patent literature. All progressive lens optics consist of two refracting surfaces. The anterior surface contains a non-spherical shape which provides the different powered regions, and the posterior surface is either a pure sphere or a toric which provides the base power or provides the base power and corrects for the user's astigmatism respectively. The front surface geometry is optimized to minimize unwanted astigmatism and other aberrations, provide a first optical zone with minimal astigmatic aberrations for viewing distant objects, a second optical zone with a higher spherical power for viewing near objects, and a third optical zone connecting the first two zones, of variable spherical power for providing intermediate vision. In an early design, shown in U.S. Pat. No. 2,878,721, the unwanted astigmatism was spread out over the entire optic, thus reducing the peak astigmatism. But the connecting intermediate zone does not provide a smooth transition in refractive power, and the residual unwanted astigmatism in the distance and the near viewing zones remains unacceptably high. In more recent progressive designs, the zones designed for distance and near vision are kept spherical, while the unwanted astigmatism which inevitably comes from connecting the distance and near zones with a smooth, continuous surface is (1) spread over as large an area of the optic as possible in order to reduce the maximum unwanted astigmatism, (2) spread out more evenly. See, for example, U.S. Pat. Nos. 4,056,311 and 4,315,673. In all cases, the posterior surface of the optic is left spherical or toroidal, and is not designed to provide correction of unwanted astigmatic error caused by a progressive lens surface itself. As a result, even after fifty years of research and development, state of the art progressive addition lenses have numerous shortcomings, including high levels of peripheral astigmatism, significant peripheral refractive errors, narrow channel width, as well as insufficient widths of the near and distance power zones, limiting peripheral vision. Design of ophthalmic lens optics eschew optimization of the posterior surface, because the conventional method of fabrication of progressive addition lens optics precludes development and specification of complex geometries for the posterior surface, as will be made clearer in the subsequent section. Indeed, the aspheric correction provided to minimize the unwanted astigmatism inherent in single vision lenses, are also confined to the front surface of the optic.
All of these methods of dealing with unwanted astigmatic errors involve managing the error by distributing it over larger or remote areas of the lens or other such techniques. None actually eliminate the astigmatic error.
Previously known methods of fabrication of ophthalmic lens optics may have so far prevented optics designers from providing aspheric corrections to the posterior surface of the optic, and also from proposing multilayer optics incorporating intermediate or "buried" surfaces. The method of fabrication of ophthalmic optics begins with the casting a semi finished blank from an optical material which incorporates the front (anterior) surface of the optic in finished form, often coated with a scratch resistant layer. This semi finished blank is subsequently custom ground and polished in regional grinding laboratories, or occasionally in retail locations to fit a particular prescription. The posterior surface is thus rendered spherical or toroidal, depending on the prescription, the axis of the cylinder correction being set by placing the principal meridian passing through the optical center of the distance power zone and the optical center of the near power zone at the appropriate angular orientation with reference marks on the mounting fixture used to hold the blank during the machining process. For the most part, the grinding and polishing equipment used in finishing labs is only capable of providing spherical or toroidal curves, hence for the ophthalmic optics to be available widely, the optics designer can not rely upon the posterior surface to be aspheric or corrective.
There is thus a need to develop designs for ophthalmic lens optics in which the posterior surface of the optic and/or intermediate surfaces are designed to eliminate as nearly as possible the unwanted astigmatism induced by the continuous change of the radius of curvature of the front surface required for intermediate vision, and to provide methods of their fabrication.